Metal Ion-Engineered Carbon Quantum Dots From Hazelnut Shell via Solid-State Synthesis for Efficient OLED Devices

In this study, we report a scalable and green solid-state synthesis of nitrogen-doped carbon quantum dots (CQDs) from hazelnut shell biomass, using citric acid and urea as carbon and nitrogen sources. Controlled BaCl₂ and ZnCl₂ doping was applied to tailor nucleation, crystallinity, and surface chemistry. Structural analyses (FTIR, XRD, STEM, XPS, and DLS) revealed that BaCl₂-assisted CQDs exhibited higher graphitization, narrower size distribution (7-13 nm), and fewer defects, while ZnCl₂-assisted CQDs showed more amorphous and heteroatom-rich surfaces. Optical measurements indicated strong π-π* absorption (≈280 nm), bright blue emission (λ_em 405-412 nm), and quantum yields of 63.4% (BaCl₂) and 50.1% (ZnCl₂), with > 95% stability after 30 days. When used as OLED emissive layers, BaCl₂-CQDs achieved a luminous efficiency of 0.75 cd A^-1, nearly four times that of ZnCl₂-CQDs (0.20 cd A^-1), despite lower maximum luminance (48.9 vs. 308.1 cd m^-2). These results highlight metal ion-assisted nucleation as an effective strategy to engineer CQD properties and enhance device performance, paving the way for sustainable, scalable OLED technologies.